This isn't my experience. I lived in a few different suburbs and there were always sidewalks. The only place I've been to where I really noticed their absence is smaller cities in New England. I always assumed it was due to the fact that the weather precluding walking so much of the time anyway.
And of course in other places in the world where cities are old and streets are too small for sidewalks. Back streets in such places are comparable to alleys in the US.
It's only due to the local governments not requiring developers to build sidewalks. Most modern cities require any new or modifications to implement sidewalks.
The Northeast has a lot of old places that you will see small strips of sidewalk and then no sidewalk, and then sidewalk again. Because they force the new construction to have it, but the stuff that was already there won't.
When I was consulting in Oaks PA I tried to walk from my hotel to the local pub. It wasn't particularly far, but walking along unlit and unpaved tracts of land wasn't particularly appealing.
I would have walked if there were any affordances at all, but ended up driving more often than not.
And of course in other places in the world where cities are old and streets are too small for sidewalks. Back streets in such places are comparable to alleys in the US.